a) Field of the Invention
The present invention relates to a thin film magnetic head and a method of manufacturing the same, capable of reducing dips of a reproduction signal and improving the reproduction characteristics.
b) Description of the Related Art
A thin film magnetic head is used as a recording/reproducing means of a magnetic disc drive. A conventional magnetic head used by a magnetic disc drive is shown in FIGS. 2A to 2C. FIG. 2A is a front view of the magnetic head, FIG. 2B is a cross sectional view taken along line 2B--2B in FIG. 2A, and FIG. 2C shows a pole end surface as viewed from line 2C--2C. In this example, a coil is made of three coil layers 20, 24, and 28.
The thin film magnetic head 1 is formed on a slider substrate 10 having a polished clean mirror surface. The slider substrate 10 is made of, for example, a ceramic plate of Al.sub.2 O.sub.3 --TiC. A protection layer 12 made of inorganic insulating material such as SiO.sub.2 and Al.sub.2 O.sub.3 is deposited on the substrate 10 to a thickness of several tens .mu.m by sputtering. A lower magnetic film 14 is laminated upon the protection layer 12 by electroplating. A magnetic gap layer 16 is laminated (deposited) upon the lower magnetic layer 14 by sputtering, the magnetic gap layer 16 forming a magnetic gap 17 at the pole portion of the magnetic head. The magnetic gap layer 16 is made of such as SiO.sub.2 and Al.sub.2 O.sub.3 like the protection layer 12.
A first insulating layer 18 is laminated upon the magnetic gap layer 16. This insulating layer 18 is usually made of positive photoresist thermally cured and stabilized. On the first insulating layer 18, a first coil layer 20 made of Cu or other metals is deposited by electroplating to a thickness or several .mu.m. On the first coil layer 20, a second insulating layer 22, a second coil layer 24, a third insulating layer 26, a third coil layer 28, and a fourth insulating layer 30 are sequentially laminated in this order by similar methods as above.
On the fourth insulating layer 30, an upper magnetic layer 32 is formed by electroplating. A throat height TH is defined by the portion where the upper and lower magnetic layers 32 and 14 face in parallel. With the magnetic gap 17 being interposed therebetween. The end portion 90 of the upper magnetic layer 32 opposite to the pule side is in tight contact with the lower magnetic layer 14. A passivation layer 34 is formed on the whole surface of the upper magnetic layer 32 by sputtering.
As shown in FIG. 2C, the exposed pole surfaces of the upper and lower magnetic layers 14 and 32 of the magnetic head 1 have leading and trailing edges 40 and 42 of lower (leading) and upper (trailing) poles 36 and 38. The leading and trailing edges 40 and 42 are linear and parallel to the magnetic gap 17. The thin film magnetic head 1 moves relative to a magnetic recording medium, and the leading pole 40 tracks the medium first and the trailing pole 30 follows thereafter.
A reproduction output waveform of a magnetic head having the edge shape shown in FIGS. 2A to 2C is illustrated in FIG. 3. The abscissa of the waveform represents a reproduction position, and the ordinate represents a reproduction signal voltage. A solitary wave having a peak voltage V.sub.L appears at the reproduction position where the magnetizing direction of a recording medium is inverted. In addition to this output of a voltage V.sub.L, dips (undershoots) d1 and d2 are generated. The dip d1 is generated because the thickness P1 of the leading (lower) pole 36 is finite, and the dip d2 is generated because the thickness P2 of the trailing (upper) pole 38 is finite. The larger these dips d1 and d2, the more errors are likely to be generated in a PRML (partial response maximum likelihood) signal process. As shown in FIG. 4, in the PRML, signal process, a signal whose dips are cut is generally used. In order to cut the dips, a signal is also required to be cut. Therefore, the larger the dips, the more the signal is required to be cut. This leads to a smaller SN margin and a larger probability of errors.
In order to reduce dips, a pole shape such as shown in FIG. 5 has been proposed. Four corners of each pole 36, 38 are trimmed from the pole end surface by ion etching or the like. With this pole shape, however, the following disadvantage occurs. After a wafer is formed with a number of thin film magnetic heads, this wafer is required to be cut into rows (rectangles) so that after the cut surface of each row is processed to realize a predetermined throat height, the four corners of each pole 36, 38 are trimmed by photolithography or etching. The four corners of each pole 36, 38 cannot be trimmed after the wafer is cut into rows, because the end surface of the pole 36, 38 exposes not on the wafer surface side but on the row cut side. Therefore, each row is required to be processed independently so that productivity becomes low.
Other pole shapes reducing dips have been proposed by the assignee of the present invention, in the embodiments of Japanese Patent Application No.3-81458 now laid open application JP-A 4-356704. The cross sections of these pole shapes are shown in FIGS. 6A to 6D. Since processes of realizing these pole shapes can be performed on the wafer surface side, productivity is better than the process to be performed on the pole end side as illustrated in FIG. 5.
Among the pole shapes shown in FIGS. 6A to 6D, the pole shapes shown in FIGS. 6A and 6B are thickened with a thickness P1 toward one of or both sides of the lower (leading) edge 36 of the lower pole 36. Therefore, the off-track overwrite performance for adjacent tracks is deteriorated. Therefore, a distance between adjacent tracks is required to be widened, which results in a low record density. In the case of a leading edge 40 of a protruding arc shape shown in FIG. 6C, it is necessary to form an underlying layer with a recess of an arc cross section. It is not easy to form such a downward arc recess on the surface of the underlying layer or protection layer 12 (FIGS. 2B and 2C) by photolithography or etching. Only one method is to mechanically form such a curved recess and the productivity is lowered.
In contrast with the above pole shapes, a leading edge 40 shown in FIG. 6D is trapezoidal and has a central portion of the lower pole 36 with a thickness P1 and opposite end portions with a thickness P1' thinner than P1. This pole shape has a good off-track overwrite performance for adjacent tracks because the opposite end portions of the lower pole 36 have a thinner thickness P1'. In addition, since this leading edge has a straight line shape, it can be realized by photolithography or etching, and the productivity is good.
However, even the leading edge shape shown in FIG. 6D may increase wiggles and degrade the overwrite performance, depending upon the dimensions of the leading edge shape.